Process for secondary recovery

ABSTRACT

Hydrocarbons are recovered from a subterranean hydrocarbon-bearing formation penetrated by an injection well and a production well by displacing hydrocarbons toward the production well using a drive fluid such as water thickened with a copolymer of acrylamide and vinyl sulfonic acid or salts thereof or with a copolymer of acrylamide and styrene sulfonic acid or salts thereof as well as these same copolymers alkoxylated with an alkylene oxide. Optionally, the drive fluid can be saturated with carbon dioxide, nitrogen, natural gas or mixtures of these gases.

This is a division of application Ser. No. 233,438, filed Feb. 11, 1981,now U.S. Pat. No. 4,343,712, which is a continuation-in-part ofapplication Ser. No. 75,635, filed Sept. 14, 1979, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well wherein a drive fluid such as water thickenedwith a copolymer of acrylamide and vinyl sulfonic acid or the sodiumsalts thereof or with a copolymer of acrylamide and styrene sulfonicacid or salts as well as these same copolymers alkoxylated with analkylene oxide, potassium or ammonium salt thereof is utilized todisplace hydrocarbons in the formation toward a production well. Thedrive fluid employed can be saturated with carbon dioxide, nitrogen ornatural gas or mixtures thereof.

2. Prior Art

The production of petroleum products is usually accomplished by drillinginto a hydrocarbon-bearing formation and utilizing one of the well-knownrecovery methods for the recovery of hydrocarbons. However, it isrecognized that these primary recovery techniques may recover only aminor portion of the petroleum products present in the formationparticularly when applied to reservoirs of viscous crudes. Even the useof improved recovery practices involving heating, miscible flooding,water flooding and steam processing may still leave up to 70-80 percentof the original hydrocarbons in place.

Thus, many large reserves of petroleum fluids from which only smallrecoveries have been realized by present commercial recovery methods,are yet to reach a potential recovery approaching their estimatedoil-in-place.

Water flooding is one of the more widely practiced secondary recoverymethods. A successful water flood may result in recovery of 30-50percent of the original hydrocarbons left in place. However, generallythe application of water flooding to many crudes results in much lowerrecoveries.

The newer development in recovery methods for heavy crudes is the use ofsteam injection which has been applied in several modifications,including the "push-pull" technique and through-put methods, and hasresulted in significant recoveries in some areas. Crude recovery of thisprocess is enhanced through the beneficial effects of the drasticviscosity reduction that accompanies an increase in temperature. Thisreduction in viscosity facilitates the production of hydrocarbons sinceit improves their mobility, i.e., it increases their ability to flow.

However, the application of these secondary recovery techniques todepleted formations may leave major quantities of oil-in-place, sincethe crude is tightly bound to the sand particles of the formation, thatis, the sorptive capacity of the sand for the crude is great. Inaddition, interfacial tension between the immiscible phases results inentrapping crude in the pores, thereby reducing recovery. Anotherdisadvantage is the tendency of the aqueous drive fluid to finger, sinceits viscosity is considerably less than that of the crude, therebyreducing the efficiency of the processes. Another disadvantage is thetendency of the aqueous drive fluid to remove additional gas bydiffusion from the in-place oil thus further reducing the alreadylowered formation oil volume and increasing the viscosity of the oil.

There is a definite need in the art for a water flooding process inwhich the disadvantages discussed above are largely eliminated oravoided.

SUMMARY OF THE INVENTION

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well which comprises:

(A) injecting into the formation via an injection well a drive fluidcomprising water having dissolved therein a small amount of a copolymerselected from the group consisting of a copolymer of acrylamide andvinyl sulfonic acid (Polymer A) or the sodium, potassium or ammoniumsalt thereof and a copolymer of acrylamide and styrene sulfonic acid(Polymer B) or the sodium, potassium or ammonium salt thereof,

(B) forcing the said fluid through the formation and

(C) recovering hydrocarbons through the production well

In another embodiment of this invention the drive fluid may comprisewater or brine having dissolved therein a small amount, i.e., about 0.01to about 5.0 weight percent of Polymer A, B or C alkoxylated with about2 to about 150 weight percent and preferably from about 20 to about 80weight percent of a material selected from the group consisting of:

(a) ethylene oxide, and

(b) a mixture of ethylene oxide and propylene oxide wherein in the saidmixture the weight percent of ethylene oxide is about 60 to about 95.

An additional embodiment of this invention relates to the driving fluidcompositions utilized in step (A).

DETAILED DESCRIPTION OF THE INVENTION

Prior to practicing the process of this invention it is sometimesdesirable to open up a communication path through the formation by ahydraulic fracturing operation. Hydraulic fracturing is a well-knowntechnique for establishing a communication path between an injectionwell and a production well. Fracturing is usually accomplished byforcing a liquid such as water, oil or any other suitable hydrocarbonfraction into the formation at pressure of from about 300 to about 3000psig which are sufficient to rupture the formation and to open upchannels therein. By use of this method it is possible to position thefracture at any desired vertical location with respect to the bottom ofthe oil-filled zone. It is not essential that the fracture planes behorizontally oriented, although it is, of course, preferably that theybe. After the fracture has been established, and without diminishing thefracture pressure, a propping agent may be injected into the fraction inorder to prevent healing of the fracture which would destroy itsusefulness for fluid flow communication purposes. Gravel, metal shot,glass beads, sand, etc. and mixtures thereof are generally employed aspropping agents. When sand is utilized as the propping agent particleshaving a Tyler mesh size of from about 8 to about 40 are preferred(i.e., from about 0.016 to about 0.093 inches).

The copolymers of acrylamide and vinyl sulfonic acid (Polymer A)employed in this invention comprise recurring E-type units of theformula: ##STR1## and recurring F-type units of the formula: ##STR2##wherein M is selected from the group consisting of hydrogen, sodium,potassium and ammonium and wherein in the said copolymer the weightpercent of the E-type units ranges from about 65 to about 95 with thebalance being F-type units. Generally, the number average molecularweight of the acrylamide-vinyl sulfonic acid copolymers will range fromabout 10,000 to about 5,000,000 or more. The copolymers ofacrylamide-vinyl sulfonic acid and salts thereof are known materialswhich can be prepared by the usual vinyl compound polymerizationmethods. Preparation of such acrylamide-vinyl sulfonic acid polymers isdescribed in detail in Norton et al. U.S. Pat. No. 3,779,917 which isincorporated herein in its entirety by reference.

The copolymers of acrylamide and styrene sulfonic acid (Polymer B)utilized in this invention comprise recurring G-type units of theformula: ##STR3## and recurring H-type units of the formula: ##STR4##wherein M is selected from the group consisting of hydrogen, sodium,potassium and ammonium and wherein in the said copolymer the weightpercent of G-type units ranges from about 70 to about 95 with thebalance being H-type units. Generally, the number average molecularweight of the acrylamide-styrene sulfonic acid copolymers will rangefrom about 50,000 to about 5,000,000 or more. The preparation ofacrylamide-styrene sulfonic acid polymers is described in detail inJahnke U.S. Pat. No. 3,892,720 and in Jennings U.S. Pat. No. 3,804,173which are incorporated herein in their entirety by reference.

The alkoxylated copolymers of acrylamide and vinyl sulfonic acids orsalts thereof and the alkoxylated copolymers of acrylamide and styrenesulfonic acid or salts thereof useful as thickening agents in the drivefluids of this invention comprise the respective copolymers or saltsthereof alkoxylated with from about 2 to about 150 weight percent ofethylene oxide or with a mixture of ethylene oxide and propylene oxidewherein the weight percent of ethylene oxide in the said mixture isabout 60 to about 95.

Alkoxylation of the acrylamide-vinyl sulfonic acid copolymers and of theacrylamide-styrene sulfonic acid copolymers can be convenientlyconducted using methods will know in the art. For example, a solution ofthe copolymer comprising about 10 to about 30 weight percent or more ofthe copolymer in water or any other suitable solvent along with about0.5 weight percent or more of powdered potassium hydroxide or sodiumhydroxide is charged to an autoclave and the autoclave and contentsheated to a temperature of about 100° C. to about 200° C. after whichthe required weight of the alkylene oxide is pressured into the reactorwith nitrogen over a period of 1 to 3 hours or more following which theautoclave is cooled to room temperature and vented. The reaction productremaining after being stripped to remove volatile materials yields thewater-soluble, alkoxylated copolymer. In another variation, thecopolymer is reacted under pressure with a suitable glycol halohydrin toaccomplish the alkoxylation.

A number of other methods are described in the art for conducting suchalkoxylation reactions including those disclosed in U.S. Pat. Nos.2,213,477; 2,233,381; 2,131,142; 2,808,397; 3,879,475; 2,425,845 and3,062,747, etc.

In the secondary recovery process of this invention, generally theaqueous drive fluid will contain dissolved therein from about 0.01 toabout 5.0 weight percent of more of the acrylamide-vinyl sulfonic acidcopolymer, the acrylamide-styrene sulfonic acid copolymer or these samecopolymers alkoxylated with about 2 to about 150 weight percent ofethylene oxide or a mixture of ethylene oxide and propylene oxide wherethe said mixture contains about 60 to about 95 weight percent ofethylene oxide. If desired, the aqueous drive fluids previouslydescribed may be saturated with carbon dioxide, nitrogen, natural gas ormixtures thereof at the injection pressure which generally will be fromabout 300 to about 3000 psig or more. The carbon dioxide and nitrogenwhen included in the fluid, serve to reduce the viscosity of thein-place oil thus making it less difficult to displace the oil towardthe production well. The addition of natural gas to the drive fluidtends to aid in maintaining the gas saturation of the reservoir and thusprevents at least to some extent any reduction in viscosity of the oilcaused by such gas depletion.

If desired, the aqueous drive fluids having dissolved therein one ormore of the above-described polymeric thickening agents may be madealkaline by addition of an alkaline agent. The advantageous resultsachieved with the aqueous alkaline medium used in the process of thisinvention are believed to be derived from the wettability improvingcharacteristics of the alkaline agent.

Useful alkaline agents include compounds selected from the groupconsisting of alkali metal hydroxides, alkaline earth metal hydroxides,and the basic salts of the alkali metal or alkaline earth metals whichare capable of hydrolyzing in an aqueous medium to give an alkalinesolution. The concentration of the alkaline agent employed in the drivefluid is generally from about 0.005 to about 0.3 weight percent. Also,alkaline materials such as sodium hypochlorite are highly effective asalkaline agents. Examples of these especially useful alkaline agentsinclude sodium hydroxide, potassium hydroxide, lithium hydroxide,ammonium hydroxide, sodium hypochlorite, potassium hypochlorite, sodiumcarbonate and potassium carbonate.

A wide variety of surfactants such as linear alkylaryl sulfonates, alkylpolyethoxylated sulfates, etc. may also be included as a part of theaqueous drive fluid compositions previously described. Generally about0.001 to about 1.0 or more weight percent of the surfactant will beincluded in the drive fluid.

This invention is best understood by reference to the following examplewhich is offered only as an illustrative embodiment of this inventionand is not intended to be limitative.

EXAMPLE I

In a field in which the primary production has already been exhausted,an injection well is completed in the hydrocarbon-bearing formation andperforations are formed between the interval of 9860-5875 feet. Aproduction well is drilled approximately 430 feet distance from theinjection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 5845-5880 feet.

The hydrocarbon-bearing formation in both the injection well and theproduction well is hydraulically fractured using conventionaltechniques, and a gravel-sand mixture is injected into the fracture tohold it open and prevent healing of the fracture.

In the next step water saturated with nitrogen at a pressure of about1320 psig at a temperature of 60° F. to which there has been added about0.14 weight percent sodium hydroxide and containing dissolved therein0.32 weight percent of an acrylamide-styrene sulfonic acid copolymerhaving a number average molecular weight of about 280,000 alkoxylatedwith about 25 weight percent of ethylene oxide prepared in the mannerpreviously described above is injected via the injection well into theformation at a pressure of 1320 psig and at the rate of 1.15 barrels perminute. In the acrylamide-styrene sulfonic copolymer the weight percentof the acrylamide units is about 85 with the balance being styrenesulfonic acid units. Injection of the driving fluid is continued at therate of about 1.15 barrels per minute and at the end of 78 days the rateof production of oil is substantially greater than with water injectionalone.

EXAMPLE II

An injection well is drilled and completed in the hydrocarbon-bearingformation of a Berea field in which primary production has beenexhausted and perforations are formed between the interval of 6125-6142feet. A production well is drilled approximately 410 feet distance fromthe injection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 6128-6145 feet.

The formation in both the injection well and the production well ishydraulically fractured using conventional techniques and employing theusual gravel-sand mixture as a propant to hold open the fracture.

Next, water saturated with carbon dioxide at a pressure of about 1150psig at a temperature of 65° F. and containing dissolved therein 0.61weight percent of an acrylamide-sodium vinyl sulfonate copolymer havinga number average molecular weight of about 820,000 prepared in themanner described in Norton et al. U.S. Pat. No. 3,779,917 and alsocontaining dissolved therein 0.20 weight percent of anacrylamide-styrene sulfonic acid copolymer having a number averagemolecular weight of about 139,000 alkoxylated with about 65 weightpercent of a mixture consisting of ethylene oxide (87 weight percent)and propylene oxide (13 weight percent) is injected via the injectionwell into the formation at a pressure of about 1150 psig and at the rateof 1.2 barrels per minute. In the acrylamide-styrene sulfonic acidcopolymer the weight percent of the acrylamide units is about 90 withthe balance being the styrene sulfonic acid units. Injection of thedriving fluid is continued at the rate of 1.05 barrels per minute and atthe end of 85 days the rate of production of oil is substantiallygreater than with water injection alone.

In some instances it may be desirable to include in the drive fluid anagent to prevent degradation of the thickening agent, i.e., thecopolymers previously described by bacterial action. Therefore, ifdesired, the drive fluid may contain along with the acrylamide-vinylsulfonic acid copolymer, the acrylamide-styrene sulfonic acid copolymeror oxyalkylated derivatives thereof or any other suitable thickeningagent from about 0.01 to about 5.0 weight percent or more of, forexample, a water-soluble terpolymer comprising recurring E-type units ofthe formula: ##STR5## recurring F-type units of the formula: ##STR6##and recurring G-type units of the formula: ##STR7## wherein M isselected from the group consisting of hydrogen, sodium, potassium andammonia and wherein in the said terpolymer the weight percent of theE-type units ranges from about 60 to about 95, the weight percent of theG-type units ranges from about 2 to about 10 and with the balance beingthe F-type units. Generally, the number average molecular weight of theabove-described terpolymers will range from about 50,000 to about8,000,000 or more. These terpolymers can be prepared by the usual vinylcompound polymerization methods. Such methods are more completelydescribed in detail in Norton et al. U.S. Pat. No. 3,779,917 and inJahnke U.S. Pat. No. 3,892,720 both of which patents are incorporatedherein in their entirety by reference. These same terpolymers may beemployed as the sole thickening agent in the drive fluid. Also, theabove-described acrylamide-styrene sulfonic acid or sodium, potassium orammonium salt thereof can be alkoxylated, i.e., reacted with about 2 toabout 150 weight percent of ethylene oxide or with a mixture of ethyleneoxide and propylene oxide wherein the weight percent of ethylene oxidein the mixture ranges from about 60 to about 95 to form an alkoxylatedterpolymer useful by itself or with other thickening agents in amountsof from about 0.01 to about 5.0 weight percent in the drive fluids ofthis invention. The alkoxylation of the terpolymers can be conducted inthe same manner as previously described for the alkoxylation of thecopolymers.

The following example illustrates the preparation of an alkoxylatedacrylamide-sodium styrene sulfonate-vinyl chloride terpolymer.

EXAMPLE III

A total of 430 cc of water, 5 g. of powdered potassium hydroxide and 58g of polyacrylamide-sodium styrene sulfonate-vinyl chloride terpolymer(number average molecular weight of about 2,000,000, 80 weight percentacrylamide, 5 weight percent vinyl chloride and 15 weight percent sodiumstyrene sulfonate) are added to an autoclave which is then heated to atemperature of 130° C. Ethylene oxide in the amount of 63 g. is added tothe autoclave under nitrogen pressure over a 1.1 hour period duringwhich time the temperature of the autoclave is maintained is 130° C.Next, the autoclave and contents are allowed to cool to room temperatureafter which the autoclave is vented. The reaction mixture is thenstripped of volatiles using a nitrogen purge. The resultingwater-soluble product is the acrylamide-sodium styrene sulfonate-vinylchloride terpolymer alkoxylated with about 90 weight percent of ethyleneoxide.

EXAMPLE IV

In a field in which the primary production has already been exhausted,an injection well is completed in the hydrocarbon-bearing formation andperforations are formed between the interval of 5050-5070 feet. Aproduction well is drilled approximately 415 feet distance from theinjection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 5055-5075 feet.

The hydrocarbon-bearing formation in both the injection well and theproduction well is hydraulically fractured using conventionaltechniques, and a gravel-sand mixture is injected into the fracture tohold it open and prevent healing of the fracture.

In the next step water saturated with carbon dioxide at 1210 psig at atemperature of 70° F. containing dissolved therein about 0.74 weightpercent of a nonionic surfactant of the formula: ##STR8## and containingdissolved therein 0.50 weight percent of an acrylamide-sodium styrenesulfonate-vinyl chloride terpolymer (78 weight percent acrylamide, 17weight percent sodium styrene sulfonate and 6 weight percent vinylchloride-number average molecular weight of about 2,850,000) alkoxylatedwith about 48 weight percent of ethylene oxide prepared in the mannerpreviously described in Example III above in injected via the injectionwell into the formation at a pressure of about 1210 psig and at the rateof 1.2 barrels per minute. Injection of the driving fluid is continuedat the rate of about 1.2 barrels per minute and at the end of 83 daysthe rate of production of oil is substantially greater than with waterinjection alone.

What is claimed is:
 1. A copolymer of acrylamide-styrene sulfonic acidor the sodium, potassium or ammonium salt thereof, alkoxylated with amaterial selected from the group consisting of ethylene oxide or amixture of ethylene oxide and propylene oxide.
 2. The copolymer of claim1 wherein the weight percent of acrylamide derived units ranges fromabout 70 to about 95 with the balance being styrene sulfonic acidderived units.
 3. The copolymer of claim 1 wherein the average numbermolecular weight of the acrylamide-styrene sulfonic acid copolymer willrange from about 50,000 to about 5,000,000.
 4. The copolymer of claim 1,alkoxylated with about 2 to about 150 weight percent of ethylene oxide.5. The copolymer of claim 1, alkoxylated with about 2 to about 150weight percent of a mixture of ethylene oxide and propylene oxide,wherein the weight percent of ethylene oxide in the said mixture isabout 60 to about 95.